The present invention relates to a watchcase, and is more specifically concerned with the assembly of two parts of said case, at least one of them transparent and visible from the outside of the case.
There are several methods commonly adopted to achieve such an assembly. The first is that in which the transparent part, for example the crystal, is held to another part of the case, for example the bezel or the body, by purely mechanical means exerting a geometrical locking action on the crystal (clips, tracks, rims resting on a peripheral tongue of the crystal, screws) or by friction (setting of the crystal in a recess of the case).
These methods share the disadvantages of requiring either special structures on the case, not always compatible with the current tendency towards reduction of case thickness, or operations of drilling or machining the crystal, which are especially difficult to carry out on sapphire crystals used for quality watchcases. Besides, in all these assemblies it is necessary to provide sealing means, such as O-rings, independently of the means providing mechanical retention.
It will be seen that all these considerations subject the designer of watchcases to technical constraints that greatly limit the scope in creating new styles. For example, the means described above will allow cases to be made in which the crystal completely covers the top margin of the body on all sides without being held there by clips, pins, screws or other accessories encumbering the lines of the case.
Recently, therefore, a different approach has been developed, in which the crystal is held to another part of the case by means of a layer of cement. A construction using this technique is described in Swiss Patent Application No. 622,151.
Although that method serves to avoid the above mentioned disadvantages, it introduces limitations of its own, deriving directly from the properties of the cement. Specifically, the case of the above mentioned application can be built only if the body and the crystal have similar hardnesses and coefficients of expansion (sapphire on hard alloy, plastic on plastic etc) to avoid damage to the layer of cement in the event of variations in the temperature of the case.
Besides, it is difficult to obtain cements whose properties of seal and mechanical strength will both remain stable over time. To overcome this drawback, Japanese Utility Design No. 1,243,579 proposes the use of two layers of different cements juxtaposed, one to provide strength of the assembly, and the other a tight seal.
It will also be noted that for a satisfactory assembly, it is necessary to provide a comparatively great width of the layer of cement. Since the cement is generally not transparent, or does not remain so, it must be masked by metalizing the surface of the glass in the area of contact with the cement, the not inconsiderable width of which will lend an appearance of heaviness to the case.
It has likewise previously been proposed that the crystal be assembled with the bezel or body of a watchcase by means of a low-melting-point soldering material.
Thus Swiss Pat. No. 582,909 describes constructions in which the solder is applied directly between the wall of the watchcase and the surface of the crystal. To allow the soldering material to be used with the crystal, it is specified that substances such as zinc or rare earth metals are to be incorporated with the said material. No such products have ever appeared on the market. It may be supposed that they have given rise to many difficulties as to endurance of the solder-crystal junction, which would certainly be shortened by phenomena of oxidation of the welding material. Furthermore, it is clear that the appearance of such solders, visible through the crystal, would militate against their use in quality watches.
The use of a low-temperature soldering material is likewise suggested in Belgian Pat. No. 688,991. That patent, which relates to the installation of a watch movement under vacuum, specifies that the crystal, its base having previously been electrochemically coated with platinum, gold, silver or copper, is set in a recess without undue clearance. In the back corner of this recess, a small seam of low-temperature solder is laid, to be afterwards melted. In this arrangement, it is clear that the small fillet of solder in the corner of the recess serves essentially to seal the case. The mechanical retention of the crystal is provided by the recess in which it must be fitted and by the negative pressure created inside the watchcase. Thus the patent does not suggest effecting the mechanical assembly of a crystal to a watchcase with the help of solder alone. Besides, the materials deposited on the crystal are characterized by excellent permeability to diffusion of the soldering material, and therefore thick layers of this must be provided if the solder is not to show through the crystal. Now thick electrochemical deposits on mineral glass cannot provide enough adhesion to ensure adequate anchorage of the crystal unless it is held by other means.
Hence one of the essential objects of the invention is to propose means actually permitting use of a low-temperature soldering material as the sole means of assembling the crystal with a watchcase. For this end, and in view of the esthetic as well as mechanical requirements stated above, it has been found highly desirable to provide a metallic coating of special structure on the portion of the crystal surface facing the layer of solder. In general, said coating may be defined as a combination of at least three layers, some or even all of which, incidentally, may consist of the same material.
The first layer, in contact with the surface of the crystal, serves both for proper adhesion of the coating to the crystal and for masking the soldered area. The second layer serves to arrest diffusion of soldering material towards the crystal. Lastly, the third layer, at the interface between coating and solder, on the contrary permits ready diffusion of the latter and provides a purchase for the solder on the coating. Of course, the materials of the second and third layers as well as the method of deposition are so chosen as to provide firm adhesion to the first and second levels respectively.
Thus it is seen that the resulting assembly is effected partly by interdiffusion of elements of the soldering material and the third layer of the coating, which may be excellent and depends only on the choice of these materials, and partly from adhesion of the several layers of the coating to each other, or to the surface of the crystal, and their cohesion. This adhesion may be optimized by favorable choice of the processes of deposition employed.
In particular, a coating made of a single material such as for example gold or silver, deposited on the monocrystalline sapphire by the process of ionic deposition, may for example possess very good cohesion over a great thickness. In this case, the several layers above mentioned are made up respectively by the portion of the single thickness in contact with the crystal and the soldering material and by its medial portion, sufficiently thick to arrest diffusion of the soldering material.
Another object of the invention is to propose various constructions of watchcases the building of which is made possible by this new mode of assembly.